1 Field of the Invention
The present invention is directed to a method and system for growing highly regular crystalline structures using an electrically shielded radio frequency (ESRF) plasma source.
2 Discussion of the Background
Monocrystalline layers of silicon have been grown on silicon substrates using several techniques, including liquid phase epitaxy, vapor phase epitaxy, molecular beam epitaxy, and ion plating. The goal of epitaxial growth is to produce a grown layer that replicates the crystalline structure of the substrate on which it has grown. In addition, with regard to certain kinds of defects, an epitaxially grown layer is typically more nearly defect-free than the substrate on which it grew. Chapter 7 of Semiconductor integrated circuit processing technology, Addison-Wesley Publishing Company, Inc., Reading, Mass., 1990, by W. R. Runyan and K. E. Bean, describes epitaxy.
High resistivity layers grown epitaxially on low resistivity substrates are used to reduce the base-collector resistance and thereby increase the performance of bipolar transistors. Although transistors fabricated in MOS integrated circuits are self-isolating, higher performance and/or smaller circuit size is often possible if epitaxial techniques are incorporated in the circuit design. Consequently, epitaxial growth technology remains an important technique among the tools available to the designer of integrated circuits.
An important application of vapor phase epitaxy is the growth of high resistivity layers on low resistivity substrates. Whenever doping is achieved using either solid-state diffusion or ion implantation, the dopant is introduced from the surface; so the dopant concentration near the surface is generally greater than the dopant concentration deep inside the wafer. During growth from a melt, after a dopant has been added to the melt, it cannot, in practice, be removed; so during growth from a melt the impurity concentration may be abruptly increased, but not decreased. On the other hand, in vapor phase epitaxial growth, the dopant is deposited simultaneously with the host semiconductor (e.g., silicon), and its concentration in the epitaxially growing layer may be readily decreased or increased. Furthermore, the species of dopant can also be readily changed. Therefore, using vapor phase epitaxy, one can, in principle, sequentially grow on the same substrate uncompensated layers of p-type and n-type material having widely different resistivities.
The epitaxial layer can grow defect-free only if no solid material is formed in the gas adjacent to surface of the growing layer. That is, if a solid forming chemical reaction is involved, that reaction must be surface catalyzed. If no solid forming chemical reaction is involved, steps must be taken to insure that no groups of atoms or molecules form prior to their impinging on the growing layer.
It is known from nucleation theory that nucleation is favored at steps or ledges on a growing surface. Indeed, in many cases the observed growth rates can be explained only if a continuous supply of ledges or steps is assumed. Nevertheless, the atoms that strike the surface of the growing layer do not all suffer the same fate. Five possibilities may be identified:
1. After striking the surface, the atom may be desorbed before it becomes incorporated in the growing layer.
2. If the surface mobility of the atom is very small or if the deposition rate is very high, the atom may be surrounded by other atoms and effectively be trapped at a location that is not coincident with an appropriate crystallographic site. In such a case, a polycrystalline or amorphous layer will result.
3. If the surface mobility of the atom is great enough to permit it to move appreciably across the surface but the ledges are widely separated, it may, before reaching a ledge, join with other atoms on a terrace (i.e., a flat surface between ledges or steps) to form a stable and perhaps even properly oriented cluster that can serve as a nucleating ledge for subsequently arriving atoms.
4. If the surface mobility of the atom is great enough to permit it to move appreciably across the surface, it may diffuse to a ledge, bond properly at an appropriate crystallographic site, and thereby become a proper component of the epitaxially growing layer. For silicon and other crystalline materials characterized by the diamond lattice, a sufficient number of atomic bonds are available at a ledge or step on any crystalline surface to orient properly and bond stably any atom that diffuses to it. Ledges are most important when the depositing species is chemically hindered from forming a solid. These species have two surfaces at the ledge to remove the partially bonded atoms from the crystal forming species.
5. The atom may bond properly at an appropriate crystallographic site without having diffused to a ledge or having joined with other atoms.
In possibilities 2 through 5, the atom becomes incorporated in the growing layer and is therefore referred to as an adatom. This brief description of the several ways in which an adatom may be incorporated into the growing layer suggests that the mechanism by which such incorporation occurs will depend in a significant way upon the energy of the impinging atom. To facilitate the incorporation of adatoms at proper lattice sites, temperatures in the range from 950xc2x0 C. to 1150xc2x0 C. are usually used for the epitaxial growth of silicon on silicon. For temperatures in this range, growth rates of the order of a micrometer per minute are achieved. As the growth temperature is increased, the maximum growth rate at which a certain quality of epitaxial growth (as determined, perhaps, by defect density) can be achieved is also increased.
Interest in lower temperature procedures continues because lower processing temperatures minimize both slip defects and impurity and dopant redistribution by diffusion. Conventional epitaxial reactors have been used at temperatures in the range from 800xc2x0 C. to 1200xc2x0 C. Process temperatures in the range from 600xc2x0 C. to 800xc2x0 C. are possible using molecular beam epitaxy (MBE).
Another factor that greatly influences the quality of an epitaxially grown layer is the condition of the substrate surface. To prevent defects, such as undesired polycrystalline growth and stacking faults, from originating at the interface between the substrate and the growing layer, the substrate surface must be both damage-free and clean. Residual damage from mechanical polishing operations must be removed. One procedure for removing the damaged layer uses high-temperature HCI vapor phase etching. Standard surface preparation techniques such as the so-called xe2x80x9cRCA Cleanup,xe2x80x9d satisfactorily remove most contaminants that adversely affect subsequent epitaxial growth. However, the thin layer of oxide that remains after chemical etching, and residual carbon from adsorbed organic solvents (if epitaxial growth occurs at relatively low substrate temperatures), and any heavy metal contaminants must also be removed. The major problem, however, is the removal of the residual oxide.
Surface cleaning may also be affected with low-energy ions of an inert gas produced in a plasma. Argon is most commonly used, because it is the least expensive inert gas and the ions are heavy enough to have high sputtering yield cross-sections on most materials. For this purpose, an inductively-coupled plasma generator is especially well-suited, because it permits independent control of both plasma density and, using the independently controlled substrate bias, the energy of the impinging ions.
Finally, in cases where thermal decomposition of a silicon containing compound (e.g., SiHCI3) at the growing surface is used, other products of the reaction (e.g., chlorine) may be incorporated in the growing film and adversely affect its quality.
Various techniques for achieving single-crystal epitaxial growth of silicon on silicon have been described in the non-patent and patent archival literature. U.S. Pat. No. 3,379,584, to Bean and Runyan, entitled xe2x80x9cSemiconductor wafer with at least one epitaxial layer and methods for making samexe2x80x9d (hereinafter xe2x80x9cthe ""584 patentxe2x80x9d) describes the formation of a monocrystalline silicon epitaxial layer on a silicon substrate using thermal decomposition of silicon tetrachloride contained in a hydrogen carrier gas that also contains a small amount of antimony tetrachloride or some other suitable n-type dopant. The ""584 patent cites H. C. Theurer, xe2x80x9cEpitaxial silicon films by hydrogen reduction of SiCI4,xe2x80x9d J. Electrochemical Society, Vol. 108, pp. 649-653 (1961); and H. Christiansen and G. K. Teal, U.S. Pat. No. 2,692,839. Christiansen and Teal describe the application of vapor phase growth of germanium and silicon to the fabrication of semiconductor devices. From about 1960, the non-patent archival literature has included many articles on epitaxy. See, for example, H. C. Theurer, xe2x80x9cEpitaxial diffused transistor,xe2x80x9d Proc. IRE, Vol. 48, pp. 1642-1643, 1960.
Vacuum evaporation of silicon was described in xe2x80x9cEpitaxial growth of silicon by vacuum evaporation,xe2x80x9d Nature, Vol. 194, pp. 966-967, 1962, by B. A. Unvala. In 1966, the term molecular beam epitaxy (MBE) came into use. See B. A. Joyce and R. R. Bradley, xe2x80x9cA study of nucleation in chemically grown epitaxial silicon films using molecular beam techniques,xe2x80x9d Phil. Mag., Series 8, Vol. 14, pp. 289-299, 1966. The layers produced in early MBE experiments were of relatively poor quality due to contaminants incorporated in the growing layers. As vacuum systems improved (and became much more expensive), the quality of MBE-grown layers improved as well. However, for most purposes, MBE does not compete with other techniques for epitaxial growth. It does have advantages for special situations, however: (a) High-quality epitaxial layers can be grown at temperatures as low as a few hundreds of xc2x0 C. (b) Rapid changes in dopant concentration are possible. (c) It is well-suited for growing very thin epitaxial layers. (d) The low deposition temperature implies that the incorporation rate (the rate at which atoms can find a proper crystal site) is low, which limits the growth rate of the epitaxial layer.
Ion plating is especially notable and is described in U.S. Pat. No. 3,329,601 to D. M. Mattox, entitled xe2x80x9cApparatus for coating a cathodically biased substrate from plasma of ionized coating material,xe2x80x9d issued Jul. 4, 1967 (hereinafter xe2x80x9cthe ""601 patentxe2x80x9d). The technique described in the ""601 patent includes making a substrate to be coated the cathode of a high-voltage DC circuit, establishing a gas discharge within an evacuated reaction chamber, and evaporating a metal to be deposited into the positive glow region of the gas discharge. The surface of the substrate may be exposed to the gas discharge prior to and during the evaporation and is thereby cleaned and may be modified by ionized particles of the gas discharge. Some of the atoms to be subsequently deposited are also ionized in the gas discharge and accelerated to the substrate surface, contributing with high energy inert gas ions, and uncharged metal atoms with thermal energies. This ion bombardment produces high apparent surface temperatures without the need of bulk heating. The combination of cleaning and high energy flux at the substrate surface during deposition is conducive to the deposition of adherent metallic films and by extension to the growth of epitaxial layers, with a low density of interface defects.
D. M. Mattox""s paper entitled xe2x80x9cFundamentals of ion plating,xe2x80x9d J. Vac. Sci. Technol., Vol. 10, No. 1, pp. 47-52, January/February 1973 (hereinafter xe2x80x9cMattoxxe2x80x9d) further describes ion plating and states that:
The term ion plating is applied to atomistic film deposition processes in which the substrate is subjected to a flux of high energy ions sufficient to cause appreciable sputtering before and during film formation. Ion plating is usually done in an inert gas discharge system similar to that used in sputter deposition except that the substrate is made a high voltage sputtering cathode. The substrate is subjected to inert gas ion bombardment for a time sufficient to remove surface contaminants and barrier layers (sputter cleaning) prior to deposition of the film material. After the substrate surface is sputter cleaned the film deposition is begun without interrupting the ion bombardment. For a film to form it is necessary for the deposition rate to exceed the sputtering rate. Ion bombardment may or may not be continued after the interfacial region has been formed.
Mattox""s FIG. 3 shows a simple ion plating setup; other figures in Mattox""s U.S. Pat. No. 3,329,601 are similar. A continuously pumped vacuum system capable of attaining pressures in the 10xe2x88x927 Torr is used. The surface to be coated serves as the cathode of a diode type DC discharge; the metal deposition source, which may be a resistively heated evaporation boat, is the anode. A high-voltage DC power supply, similar to those normally used for sputtering, is used for ion plating. The gas pressure is controlled using a gas inlet with a variable leak.
Gerald W. White in U.S. Pat. No. Re. 30,401: xe2x80x9cGasless ion source,xe2x80x9d issued Sep. 9, 1980 (a reissue of U.S. Pat. No. 4,039,416), includes an RF field in his ion plating apparatus and also introduces the term xe2x80x9cgasless.xe2x80x9d Re 30,401 states that:
A gasless ion plating process wherein plating material is melted, vaporized, and then subjected to an ionization environment in a low pressure chamber with a xe2x80x9cvirtual cathodexe2x80x9d consisting of a plasma of ionized atoms of evaporant material created by evaporating in an RF field. It is a gasless ion plating process wherein the system ambient pressure prior to plating material evaporation may be much lower than that required to sustain a glow discharge, however, with vapor pressure of evaporant material added to the environment base pressure being such as to result in a plasma of ionized atoms of the plating material developing as the vaporized material approaches the RF cathode.
Gerald W. White and Jack C. Volkers describe in U.S. Pat. No. 4,342,631: xe2x80x9cGasless ion plating process and apparatus,xe2x80x9d issued Aug. 3, 1982, a gasless ion plating apparatus that employs RF energy.
In accordance with a disclosed embodiment, one or more substrates to be plated are placed within the chamber. Also within the chamber there is disposed a plating source which includes plating material. The chamber is evacuated and the plating material is heated to evaporate the plating material. Radio frequency energy is applied to the plating source to form a plasma of positively charged plating ions from the vaporized plating material. A positive direct current bias is developed on the plating source relative to the substrates by, for example, applying a direct current positive voltage to the plating source to create an electrical field between the source and substrates for accelerating the plating ions towards the substrates for plating same.
The White patents describe an RF plasma apparatus that is essentially a capacitively-coupled plasma generator. However, neither Mattox (U.S. Pat. No. 3,329,601), White (U.S. Pat. No Re. 30,401), nor White and Volkers (U.S. Pat. No. 4,342,631) discusses the production of epitaxial single-crystal layers.
It is an object of the present invention to provide an improved growth technique as compared to the prior art. Measures of value for a particular epitaxial growth technique include, but may not be limited to: (a) defect density and doping uniformity in the epitaxial layer, (b) throughput or deposition rate, (c) ability to change dopants and/or dopant concentration rapidly as growth proceeds or between runs, (d) potential capability of process and equipment to be adapted to production of larger wafers, and (e) cost per wafer. The present invention provides advantageous results for criteria (a)-(d), and may also represent an improvement with regard to criteria (e).
It is another object of the present invention to grow epitaxial single crystalline layers (e.g., silicon layers) on a substrate (e.g., a silicon substrate) in an ESRF source using the techniques of gasless ion plating since the low energy of the impinging ions/atoms does not cause appreciable damage to either the substrate or the growing layer.
The above objects and other advantages of the present invention are made possible by: a process chamber and an associated vacuum pumping system capable of maintaining a pressure of 1 m Torr or less in the chamber during the epitaxial growth of an atomic or molecular layer on a substrate; an evaporation source capable of producing the atomic or molecular vapor; a source of an inert gas and an adjustable valve through which the inert gas may be admitted to the process chamber; an electrically shielded radio frequency (ESRF) plasma source having wall-bias capability; and a substrate support (chuck) with an extremely low radio frequency (RF) impedance drive system.